http://www.mamapedia.com/questions/17625482087945666561
This was my post about my daughter from the other day, in a similar situation. I know it's titled "girl drama" but really, I think it's more "pre-schooler drama" and what starts to happen when they are 4 or 5 and in school for the first time and starting to have more friendships with their peers - they are only just figuring out what friendship is all about. My daughter wants to be friends with another girl in her class, but this other little girl tends to run hot and cold - one day she's DD's "best friend" and then another day she's in a bad mood and just wants everyone to leave her alone. And DD can't always figure it out and not take it personally. I've tried to explain to her, "It's not you, it's her, and don't worry about it." And that while she can be nice to everyone, not everybody is going to be her "best friend" and they don't have to be either.
I actually learned more about this girl in particular by talking with the teachers. Not that they can always say very much, because sometimes it could be something really personal, but basically that this is her personality - she gets upset about something and will carry that bad attitude around with her all day. And that's hard for DD to understand, because she's not like that at all - she gets over stuff pretty quickly and just wants to be friends with everyone. Of course, she could be friends with the other 18 kids in the class but if this 1 girl decides she doesn't want to give her the time of day, you would think she doesn't have any friends at all. I've been talking to her about the fact that she can try to cheer this other girl up if she's being pouty but if she's just being mean, to just leave her alone and find someone else to play with.
Since you know the boy's mom, if you feel comfortable talking about it, I would just take a very low-key non-confrontational approach and see if you guys can figure out what is really going on. Talking with the teacher might be insightful too. I agree that it's good to let kids work things out in their own, but at this age, they still need help - sometimes these are rough murky waters that can be tough to navigate when they only have a very young child's perspective.